Apparatus and method for reducing the forces on extendible legs of a floating vessel

ABSTRACT

Semi-submersible legs are embodied as close-ended cylinders or other similar shapes or combination of shapes having the capability to be ballasted or made buoyant. The semi-submersible legs are connected to a jack-up drilling vessel so that they slide vertically through the vessel deck into the underlying body of water and are lockable at any position along their vertical path. The method of their use requires lowering the extendible main supporting legs of a jack-up drilling vessel above the ocean floor, lowering the semi-submersible legs into the water, making them buoyant so as to cause the vessel to raise out of the water to a distance above probable wave action at which time the extendible main support legs are lowered to a supporting position onto or into the ocean floor. With the extendible supporting legs in a supporting position, the semi-submersible legs are retracted; if the extendible legs penetrate a soft bottom, the semi-submersible legs are first ballasted to serve as an additional jacking down source for the supporting legs and then retracted and deballasted. The foregoing steps are reversed when the drilling vessel is to be relocated with or without the supplemental steps of making the semi-submersible legs additionally buoyant and or lowering the vessel hull to the water surface thus providing an extra lift force on the support legs. The vessel hull is jacked up on the buoyant support of the semisubmersible legs prior to completely removing the extendible main legs from the ocean floor penetration or surace contact.

United States Patent [19] Fischer 1 Mar. 25, 1975 [54] APPARATUS ANDMETHOD FOR REDUCING THE FORCES ON EXTENDIBLE LEGS OF A FLOATING VESSEL[75] Inventor: William Fischer, Fullerton, Calif.

[73] Assignee: Chevron Research Company, San

Francisco, Calif.

[22] Filed: Dec. 21, 1973 [21] Appl. N0.: 427,175

[52] US. Cl. 61/465, 114/.5 D

[51] Int. Cl. E02b 17/00, B63b 35/00 [58] Field of Search 61/465, 65;114/.5 D; 175/7, 8, 9

[56] References Cited UNlTED STATES PATENTS 2,771,747 11/1956 Rechtin61/465 2841961 7/1958 Lucas 61/465 3.605669 9/1971 Yu 61/465 X PrimaryExaminer-Jacob Shapiro Attorney, Agent, or FirmRalph L. Freeland, .lr.;Robert T. Kloeppel [57] ABSTRACT Semi-sumbersible legs are embodied asclose-ended cylinders or other similar shapes or combination of shapeshaving the capability to be ballasted or made buoyant. Thesemi-submersible legs are connected to a jack-up drilling vessel so thatthey slide vertically through the vessel deck into the underlying bodyof water and are lockable at any position along their vertical path. Themethod of their use requires lowering the extendible main supportinglegs of a jack-up drilling vessel above the ocean floor, lowering thesemisubmersible legs into the water, making them buoyant so as to causethe vessel to raise out of the water to a distance above probable waveaction at which time the extendible main support legs are lowered to asupporting position onto or into the ocean floor. With the extendiblesupporting legs in a supporting position, the semi-submersible legs areretracted; if the extendible legs penetrate a soft bottom, thesemi-submersible legs are first ballasted to serve as an additionaljacking down source for the supporting legs and then retracted anddeballasted, The foregoing steps are reversed when the drilling vesselis to be relocated with or without the supplemental steps of making thesemisubmersible legs additionally buoyant and or lowering the vesselbull to the water surface thus providing an extra lift force on thesupport legs. The vessel hull is jacked up on the buoyant support of thesemisubmersible legs prior to completely removing the extendible mainlegs from the ocean floor penetration or surace contact.

12 Claims, 8 Drawing Figures :::::gi l

PATENTEI] HARE 51975 sums or 5 APPARATUS AND METHOD FOR REDUCING THEFORCES ON EXTENDIBLE LEGS OF A FLOATING VESSEL BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to marinestructures. Specifically, this invention is related to a new componentof a mobile drilling vessel which reduces the destructive forces onextendible support legs during the following intervals: when legs aremoved from a stowed position on the drilling vessel into a loadsupporting position into or on the ocean floor and especially during theinterval when the support legs are near the ocean floor while the vesselis subject to rough water conditions. The invention is also used whenthe supporting legs are removed from the bottom and put into a stowedposition.

2. Description of Prior Art Offshore drilling vessels which areself-propelled or towed on the water surface from one location toanother have been used in the past. One type of such vessel is supportedand jacked clear of the water surface on extendible legs that arelowered to the water bottom when a site is reached. In this jacked upposition the vessel becomes a stable drilling platform. However, theextendible legs have experienced much damage, including ultimate failurein some instances, due to the legs ramming against and/or rippingthrough the under water bottom in even relatively calm water; that is,waves between 3 to 5 feet. In some offshore areas like the Gulf ofAlaska or the North Sea, the wave heights are rarely below the safelimit for the transfer of drilling vessel weight from the vesselsbuoyant support to the extendible leg support when moving into alocation, and the transfer from the extendible legs to the buoyantsupport of the vessel when relocating. The only alternative to gamblingon damage to the extendible legs is the common and costly practice ofawaiting calm seas. Despite this basic vulnerability in a mobile jack-updrilling vessel, many drilling companies and oil operating companiesfavor their use over other types of drilling vessels because they offervirtually the same stable support features as a permanently fixedplatform, yet being capable of relocation on short notice without thecommitment of a major capital expenditure before the extent of petroleumdeposits are ascertained.

The ramming and ripping forces mentioned above occur during the intervalwhen the legs are being located on or into the bottom or when thesupporting columns are moved off and away from the bottom in preparationfor moving the barge to another site; or more precisely, during theinterval when the total weight of the vessel transfers from the buoyantsupport of the water to a fixed support on the water or ocean floor andvice versa. During any of the foregoing times the vessel is subjected toroll, pitch and heave and combinations thereof which result inaccentuating vertical, lateral, and the combination of these motions atthe bottom of the legs relative to the ocean floor. lfthe criticalweight shift from buoyant to on-bottom support or the reverse isundertaken when wave heights exceed 3 to 5 feet, violent and sometimesdestructive forces are developed from the main legs vertically impactingand ripping through the ocean floor, at times resulting in severe legdamage.

SUMMARY OF THE INVENTION This invention is a novel way to reducedestructive forces on extendible main supporting legs of movableoffshore platforms while they are moved into or removed from a loadtransferring position; in other words, a way to permit soft touchlandings and departures of the main support legs from the ocean floor.The improvement involves the lowering of adjustably buoyantsemi-submersible legs which lift the floating platform out of the waterafter the main supporting legs are lowered and locked at a water depthprecluding any possibility of bottom contact. This sequence establishesoptimum vessel stability while the semisubmersible legs lift and/orsupport the vessel out of the water clear of probable wave action.Further, in this position, the buoyant semi-submersible legs provide aminimum motion flotation to the jack-up drilling vessel and its extendedmain legs, particularly during the critical interval when the vesselweight shifts from the buoyant support of the vessel to the bottomengaging main legs. The primary reason vessel and related main legmotion are reduced is that wave-induced fluctuations of the watersurface at the drilling site run up and down a relatively minimal waterdisplacing volume on the semisubmersible buoyant support system, thuscontributing small buoyant force changes and, therefore, small vesseland main leg movements. Whereas, when running up and down on a massiveconventional vessel hull floating on the surface of the water, this samewave action produces large buoyancy changes and therefore, large vesseland main leg movement.

With the main legs in a load carrying position on the ocean floor, thesemi-submersible legs are retracted. If, however, the main support legsrequire further penetration into the ocean floor to support futuremaximum drilling hook or other loads, a convenient loading means forseating the legs in a final supporting position is available byretracting the semisubmersible legs and ballasting them. In this way,for example, twice the total weight of the rig can be exerted as adownward force on the support legs.

When moving from a completed drilling operation, the semi-submersiblelegs are lowered a safe distance above the ocean floor, locked, and thendeballasted as required to lift the vessel hull and the main supportinglegs. If the main legs transfer their load to a hard bot-' tom, raisingthese legs presents no problem or at most requires a conventional waterpressure vacuum breaker assist below the main legs. If the legs requireddeep penetration at installation and/or the soil resistance to removalrequires more upward force to overcome than that developed by themaximum lift supplied by the semi-submersible legs and water vacuumbreaking pressure, it may become necessary to lower the vessel hull ontothe water surface for its potentially greater uplift capability. Thevessel's buoyancy is used to break free the main legs without removingthem completely from their holes. The hull is then jacked up out of thewater for safe removal of the main legs from the holes while beingsupported on its semi-submersible legs. As

soon as the legs are high enough above the water bottom to precludecontact with it, the hull is again floated. The semi-submersible legsare fully retracted and locked after which the main legs are also fullyretracted and locked. The rig is now ready to move to the next location.

The proper use of this invention will significantly reduce costlyweather downtime and costly repair operations resulting fromunsuccessful attempts to move in or move out of location in waveconditions exceeding allowable limits.

A few other examples of difficulties which this invention can alleviateis unequal platform jacking caused by jack binding, uneven waterbottoms, and driving a supporting column into a weaker soil strata thanthe other main supporting legs.

The invention also makes possible a damage control method. For example,when ajack-up platform, though safely positioned with extendible legs onthe ocean floor is struck by catastrophe, this invention can serve as anadditional safety factor which might be moved into operation, such acatastrophe is exemplified by an accidental ramming as by a large supplyvessel with sufficient force to cause damage which may lead to ultimatedestruction of one or more main legs. When this occurs, the remainingmain legs must support not only the original load, but also each mainleg must resist ad ditional moment induced by any deformation in it.Total collapse of this platform and loss of human life could occur. Insuch cases the semisubmersible legs of this invention offer someopportunity of supplementary or standby support which could mean thedifference between saving the rig and a total loss of it.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic showing an elevationof a jackup barge embodying the invention with the elements in positionfor towing the jack-up barge from one location to another;

FIG. 2 is a schematic plan view of FIG. 1;

FIG. 3 is a schematic elevation view showing the main supporting legsand semi-submersible legs partially lowered and locked in position.

FIG. 4 is a schematic elevation view showing the buoyantsemi-submersible legs lifting the working platform of the drillingvessel clear of the water and probable wave action into a motiondampening position.

FIG. 5 is a schematic elevation view showing the sup porting main legslowered in a platform supporting position before the semi-submersiblelegs are retracted.

FIG. 6 is a schematic elevation view showing the supporting columns in aplatform supporting position and the semi-submersible legs fullyretracted.

FIG. 7 is a schematic elevation showing the vessel positioned as adrilling platform with the drilling mast in an operating position.

FIG. 8 is a three dimensional view illustrating in more detail thevessel including the energy lines for jacking and locking the legs inposition.

DESCRIPTION OF THE PREFERRED EMBODIMENT EMBODIMENT Referring to FIG. 1,the mobile jack-up drilling vessel is designated 50. The equipment usedin offshore operations is mounted on the vessel deck 51 which forms theplatform 59, FIG. 4; however, only the drilling mast 52 in a stowedposition is shown. The combined weight of the vessel 50 includes theweight of the platform 59, the extendible main supporting legs 56, thesemisubmersible legs 57, and anything else on or in the vessel forwhatever reason.

Passing through the vessel hull 53 and vessel deck 51, as shown in FIGS.1 and 2, are passages which in the preferred embodiment are verticallyoriented sleeves 55. A first set of the sleeves 55 serve as verticalguides for raising or lowering of the main support legs 56 and a secondset is provided for the special semisubmersible legs 57 to serve a likefunction. The sleeves 55 are hollow tubular shaped components made fromsteel or other materials and are welded or otherwise fastened to thevessel hull 53. The semi-submersible ad justably buoyant legs 57 arehollow controllably buoyant water-tight components which can becomesufficiently buoyant to raise the vessel 50 out of the water; oralternatively ballasted so as to sink in the water. The semi-submersiblelegs 57 are made of steel or similar materials and can take the shape ofa closed ended cylinder with the end 66 which first enters the waterexpanded in the shape of a large cylinder 67 concentric with andpermanently fastened to a smaller upper cylinder 68 as shown in FIG. 3.The foregoing describes an embodiment of the semi-submersible adjustablybuoyant leg means.

When the semi-submersible legs 57 and the main supporting legs 56 are inan upright position as shown in FIG. 1, the mobile jack-up drillingvessel 50 may be surface towed or self-propelled from one location toanother. When a drilling location is reached, the main extendiblesupporting legs 56, after ballasting if required, are lowered as fardown as possible without danger of bottom contact due to hull motion asshown in FIG. 3, and locked in this position to provide optimum vesselhull stability while being lifted and supported above the water surfaceby the semisubmersible legs 57.

The semi-submersible legs 57 are controllably lowered by ballasting orjacking or a combination of the two methods and then locked in positionas shown in FIG. 3. The jacking and locking mechanism 58 which providescontrolled movement through a second set of the sleeves 55 is ofconventional design being either mechanical, hydraulic or pneumatic or acombination of these. A schematic arrangement of the control console 60,energy source 64 and lines 61 is shown in FIG. 8. In this loweredposition, shown in FIG. 4, the semisubmersible legs 57 are made buoyantthrough a blowing out process or other equivalent way. The buoyancycauses the vessel hull 53 to raise out of the water as shown in FIG. 4.In the new position, the vessel hull 53 becomes a platform 59 with thesemi-submersible legs 57 as the sole support unless the partiallylowered main legs 56 are made buoyant to assist. This new positionresults in less heave, roll and pitch to the platform 59 than if it werefloating on the water. Consequently. the shift from the buoyant supportof the semi-submersible legs 57 to the main supporting legs 56 fixed onthe bottom or a shift from the main leg support 56 to a buoyant supportis accomplished safely and without main leg 56 damage or failure inrougher waters than was possible without the semi-submersible legs 57.

The distance the vessel hull 53 is raised above the water surface isgoverned by the possible wave heights that are forecasted for theestimated duration of operations at the drilling location since waveimpact on the vessel hull 53 could cause major damage. In calm watersthis distance may be only feet or less. In rougher seas, such as theNorth Sea, it may be 30 feet or more above the water surface. In anycase, the prudent oper ator will elevate the vessel hull 53 safely abovethe highest waves expected on the drilling location over the estimatedperiod of drilling operations.

Next, the main supporting legs 56 are unlocked from their partiallylowered position, lowered or jacked down into an on-bottom position, asshown in FIG. 5, and locked. Subsequently, there is a trial deballastingof the semi-submersible legs 57 to determine whether or not any of themain legs 56 are on weak or easily compressible soil requiring furtherlowering those particular legs; otherwise, the semi-submersible legs 57are fully retracted, locked, and ballasted as required to drive and/orload test the main legs 56. The jacking and locking mechanism 54 whichprovides the controlled movement for the main support legs 56 through afirst set of sleeves 55 is shown in FIG. 8 where the control console 63,energy source 64 and lines 65 are illustrated.

With the supporting legs 56 in a platform supporting position, thevessel 50 is jacked up on the main supporting legs 56 to a safe heightabove the maximum expected wave crests; after support leg loadings areequalized they are locked into position by conventional mechanically,hydraulically or pneumatically actuated means 54 schematically shown inFIG. 8 where the control console 63, energy source 64 and lines 65 areshown. This jacking procedure not only guarantees that the deck 51 islocated at the desired elevation but also serves as a test to guaranteethat the main support legs 56 are able to support the maximum loadincurred in the drilling operations. The retracted semi-submersible legs57 are then deballasted. With the semi-submersible legs 57 retracted,FIG. 6, they do not transmit lateral loads to the main supporting legs56 from possible impact by waves lower than the maximum height expected.The drilling mast 52 is then moved in a working position as shown inFIG. 7.

The vessel hull 53, through properly coordinated manipulation'of thejacks and the locking gear 58 and the semi-submersible legs 57, ismovable up or down on the main supporting legs 56.

In case of an emergency, when one or more main supporting legs 56 becomeincapable of supporting the designated load, the adjacent or appropriatesemisubmersible legs 57 are lowered or jacked down and made to applytheir buoyancy in order to relieve the force or load which the mainsupporting leg 56 would otherwise have to transfer. After necessaryrepairs are made, if these are such as can be effected on location, thesemi'submersible legs 57 are retracted and drilling operations are againconducted.

When the vessel 50 is to be relocated, the semisubmersible legs 57 arelowered a safe distance above the water bottom, locked, and thendeballasted as required to lift the vessel hull 53 and the mainsupporting legs 56 similar to FIG. 5. If the main legs 56 transfer theirload to a hard bottom, raising these legs 56 presents little difficulty.In the case, however, where the legs 56 required deep penetration atinstallation and/or the soil comprising the underwater bottom offersmore resistance to removal than the uplift supplied by thesemi-submersible legs 57 and conventional methods of removal like watervacuum breaking pressure (not illustrated), the vessel hull 53 islowered onto the water surface to provide greater uplift capability. Theforegoing steps are used to provide just that amount of uplift forcenecessary to break free the main legs 56 without removing them fromtheir holes 69. The vessel hull 53 is then jacked up out of the waterfor removal of the main legs 56 from the holes 69 while being supportedon its semi-submersible legs 57. As soon as the legs 56 are high enoughabove the water bottom to avoid contact with it, the legs 56 are locked,FIG. 4, and the vessel hull 53 is again floated, FIG. 3. Thesemisubmersible legs 57 are fully retracted and locked after which themain legs 56 are also fully retracted and locked, FIG. I. The vessel 50is now ready to be relocated.

The foregoing description and drawings of a preferred embodiment willsuggest other embodiments and variations within the scope of the claimsto those skilled in the art, all of which are intended to be included inthe spirit of the invention as herein set forth.

I claim:

1. In a vessel adapted to float on a body of water comprising: aplatform; a plurality of passages through said platform; a plurality ofmain supporting legs operatively and adjustably connected to saidplatform; means permitting said main legs to have controlled movementthrough a first set of said passages; semi-submersible leg meansoperatively and adjustably connected to said platform for providingsufficient buoyancy to raise and support the combined weight of saidvessel above the surface of said water while said main legs are movedinto a supporting position in contact with the underwater bottom forsaid platform.

2. The vessel of claim 1 including a jacking means for raising andlowering said platform on said supporting main legs in conjunction withthe buoyant force of said semi-submersible legs when said main legs arein a platform supporting position.

3. The vessel of claim I wherein said semisubmersible means areadjustably buoyant legs.

4. The vessel of claim 1 wherein said semisubmersible leg means consistsof four semisubmersible legs operatively and adjustably connected tosaid platform; a means permitting said semisubmersible legs to havecontrolled movement through a second set of said passages; saidsemi-submersible legs providing sufficient buoyancy to raise and supportthe combined weight of said vessel above the surface of said water whilesaid main legs are moved into a supporting position.

5. The vessel of claim 3 wherein said means for lowering and raisingsaid semi-submersible legs is controlled ballasting in conjunction witha jack in order to decrease and increase the buoyancy of said legs.

6. A method of reducing destructive forces on a set of supporting mainlegs of a vessel floating on a body of water while said main legs arelowered into a position in contact with the underwater bottom comprisingthe steps of: lowering into said water a set of supporting main legs toa position above said underwater bottom so as to avoid contact with saidunderwater bottom; extending a semi-submersible controllably buoyantmeans operatively and adjustably connected to said vessel into said bodyof water; increasing the buoyancy of said means to raise said vessel toa predetermined height; positioning said main legs into a platformsupporting position in contact with said underwater bottom; andretracting said semi-submersible controllably buoyant means to saidvessel.

7. The method of claim including ballasting said main legs before saidmain legs are lowered; and making said main legs buoyant after said mainlegs are lowered.

8. A method of reducing destructive forces on a set of main supportinglegs of a vessel floating on a body of water while said main legs arelowered into a position in contact with the underwater bottom comprisingthe steps of: lowering into said water a set of supporting main legs toa position above said underwater bottom; extending a semi-submersiblecontrollably buoyant means operatively and adjustably connected to saidvessel into said body of water; increasing the buoyancy of said means toraise said vessel to a predetermined height; positioning said main legsinto a platform supporting position in contact with said underwaterbottom; ballasting said semi-submersible means so as to add additionaljacking force to said main legs and serve as a load test on said mainlegs; retracting said semisubmersible means; and deballasting saidsemisubmersible means.

9. The method of claim 7 including ballasting said main legs before saidmain legs are lowered; and making said main legs buoyant after said mainlegs are low ered.

10. A method of reducing destructive forces on a set of supporting mainlegs of a jacked up vessel located over a body of water while said mainlegs are raised into a stowed position comprising the steps of:extending a semi-submersible controllably buoyant means operatively andadjustably connected to said vessel into an underlying body of water;increasing the buoyancy of said means to support said vessel at apredetermined height; raising from said water bottom a set of supportingmain legs to a predetermined height above said bottom; lowering saidvessel to said water surface; retracting said semi-submersible means tosaid vessel; and retracting said main legs into a stowed position onsaid vessel.

11. The method of claim 9 including the additional step of furtherincreasing the buoyancy of said semisubmersible controllably buoyantmeans to provide a lifting force on said main legs.

12. A method of reducing destructive forces on a set of supporting mainlegs of a jacked up vessel located over a body of water, wherein saidmain legs are embedded in the underwater bottom, while said main legsare raised into a stowed position from said underwater bottom comprisingthe steps of: extending a semisubmersible controllably buoyant meansoperatively and adjustably connected to said vessel into an underlyingbody of water; lowering said vessel to said water surface to provide abuoyant force; increasing the buoyancy of said means to aid in looseningsaid main legs; loosening said main legs; raising said vessel to apredetermined height above said water surface on said means; raisingsaid main legs to a predetermined height above said underwater bottom;lowering said vessel to the water surface; retracting saidsemi-submersible means to said vessel; and retracting said main legsinto a stowed position on said vessel.

UNITED STATES PATENT AND TRADEMARK GFFICE CE'HFIQATE OF CDRRECTQN PATENTNO. 3,872,679

DATED March 25, 1975 V GJ WILLIAM FISCHER is certified Thai", andappears the above-Adeniified patent and tha'i said Leia; P sient arehereby caa'rected as shcwn below:

Abstract, last line, "surace" should read -surface-.

Column 3, lines 59-60, the title should read ---DESCRIPTION OF THEPREFERRED EMBODIMENT-- Signed And Scaled this twenty-sixth Day Of August1975 [SEAL] Affesf.

C. MARSHALL DANN Commissioner uflarenls and Trademarks RUTH C. MASONArresting Officer

1. In a vessel adapted to float on a body of water comprising: aplatform; a plurality of passages through said platform; a plurality ofmain supporting legs operatively and adjustably connected to saidplatform; means permitting said main legs to have controlled movementthrough a first set of said passages; semi-submersible leg meansoperatively and adjustably connected to said platform for providingsufficient buoyancy to raise and support the combined weight of saidvessel above the surface of said water while said main legs are movedinto a supporting position in contact with the underwater bottom forsaid platform.
 2. The vessel of claim 1 including a jacking means forraising and lowering said platform on said supporting main legs inconjunction with the buoyant force of said semi-submersible legs whensaid main legs are in a platform supporting position.
 3. The vessel ofclaim 1 wherein said semi-submersible means are adjustably buoyant legs.4. The vessel of claim 1 wherein said semi-submersible leg meansconsists of four semi-submersible legs operatively and adjustablyconnected to said platform; a means permitting said semi-submersiblelegs to have controlled movement through a second set of said passages;said semi-submersible legs providing sufficient buoyancy to raise andsupport the combined weight of said vessel above the surface of saidwater while said main legs are moved into a supporting position.
 5. Thevessel of claim 3 wherein said means for lowering and raising saidsemi-submersible legs is controlled ballasting in conjunction with ajack in order to decrease and increase the buoyancy of said legs.
 6. Amethod of reducing destructive forces on a set of supporting main legsof a vessel floating on a body of water while said main legs are loweredinto a position in contact with the underwater bottom comprising thesteps of: lowering into said water a set of supporting main legs to aposition above said underwater bottom so as to avoid contact with saidunderwater bottom; extending a semi-submersible controllably buoyantmeans operatively and adjustably connected to said vessel into said bodyof water; increasing the buoyancy of said means to raise said vessel toa predetermined height; positioning said main legs into a platformsupporting position in contact with said underwater bottom; andretracting said semi-submersible controllably buoyant means to saidvessel.
 7. The method of claim 5 including ballasting said main legsbefore said main legs are lowered; and making said main legs buoyantafter said main legs are lowered.
 8. A method of reducing destructiveforces on a set of main supporting legs of a vessel floating on a bodyof water while said main legs are lowered into a position in contactwith the underwater bottom comprising the steps of: lowering into saidwater a set of supporting main legs to a position above said underwaterbottom; extending a semi-submersible controllably buoyant meansoperatively and adjustably connected to said vessel into said body ofwater; increasing the buoyancy of said means to raise said vessel to apredetermined height; positioning said main legs into a platformsupporting position in contact with said underwater bottom; ballastingsaid semi-submersible means so as to add additional jacking force tosaid main legs and serve as a load test on said main legs; retractingsaid semi-submersible means; and deballasting said semi-submersiblemeans.
 9. The method of claim 7 including ballasting said main legsbefore said main legs are lowered; and making said main legs buoyantafter said main legs are lowered.
 10. A method of reducing destructiveforces on a set of supporting main legs of a jacked up vessel locatedover a body of water while said main legs are raised into a stowedposition comprising the steps of: extending a semi-submersiblecontrollably buoyaNt means operatively and adjustably connected to saidvessel into an underlying body of water; increasing the buoyancy of saidmeans to support said vessel at a predetermined height; raising fromsaid water bottom a set of supporting main legs to a predeterminedheight above said bottom; lowering said vessel to said water surface;retracting said semi-submersible means to said vessel; and retractingsaid main legs into a stowed position on said vessel.
 11. The method ofclaim 9 including the additional step of further increasing the buoyancyof said semi-submersible controllably buoyant means to provide a liftingforce on said main legs.
 12. A method of reducing destructive forces ona set of supporting main legs of a jacked up vessel located over a bodyof water, wherein said main legs are embedded in the underwater bottom,while said main legs are raised into a stowed position from saidunderwater bottom comprising the steps of: extending a semi-submersiblecontrollably buoyant means operatively and adjustably connected to saidvessel into an underlying body of water; lowering said vessel to saidwater surface to provide a buoyant force; increasing the buoyancy ofsaid means to aid in loosening said main legs; loosening said main legs;raising said vessel to a predetermined height above said water surfaceon said means; raising said main legs to a predetermined height abovesaid underwater bottom; lowering said vessel to the water surface;retracting said semi-submersible means to said vessel; and retractingsaid main legs into a stowed position on said vessel.